In a multitasking operating system, several data processing tasks are processed by the data processing system concurrently and asynchronously. For example, one or more tasks can be processed by the data processing system at the same time that the system is engaged in controlling its display. Also, in a group of several data processing systems interconnected by a network, a task may be sent for processing from the system where it originated to any other of the data processing systems. Information relating respectively to each task in progress can be displayed simultaneously in a different respective portion ("window") of a single display. (A window is a viewing area on a video display. The term is used to refer either to the full screen being used, or to a smaller region. Typically, but not exclusively, the window is rectangular in shape. Each window can have different display characteristics, such as text vs. graphics or a combination of both, colors, etc.) One graphical user interface for controlling a graphics display, designed for use with the UNIX operating system, is the "X-window", known to those in the art.
Heretofore, as a computer terminal display apparatus, a refresh-scan-type CRT (cathode ray tube) has been generally used, and a vector-scan-type CRT having a memory characteristic is sometimes used as a large, high-resolution display for CAD (computer-aided design). On a vector-scan-type CRT, an image, once displayed, is not refreshed until a subsequent screen refresh (that is, a refresh of the entire screen) is performed. For this reason, it is not well suited as a display apparatus for a real-time man-machine interfacial display, such as a moving cursor display, a moving icon display or pointing device such as a mouse, or an editorial display (for use in editing displayed text, as by insertion, deletion, moving, copying, etc. of characters or strings of characters). On the other hand, the refresh-scan-type CRT requires a refresh cycle with a frame frequency (the frequency with which the display is provided with a new picture, or frame; the frame frequency can be expressed as the reciprocal of the product of the number of scanning lines per frame and the horizontal scanning time for each line) up to 60 Hz or more for the purpose of preventing the display from flickering. A non-interlaced scanning scheme is commonly adopted so that a moving display of data in a picture, e.g., a moving display of an icon, is easy for the user to observe and follow.
With both types of CRT, the higher the desired display resolution, the larger&lt;the display apparatus becomes, resulting in higher power consumption, a larger drive controller unit and a higher cost. Such a large, high-power CRT results in various inconveniences, due to which a flat display panel has been developed in recent years.
At present, there are various systems of flat display panels. One employs a highly multiplexed drive system using super twisted nematic liquid crystals (STN). A second is a modification thereof, used for a white-and-black display. A third is a plasma display system. All of these adopt the image data transfer scheme of the CRT system and a non-interlaced scanning scheme with a frame frequency of 60 Hz or higher for screen refreshing and therefore require a total number of scanning lines on the order of 400-480 lines for one full screen. A large flat display panel having, e.g., 1000 or more scanning lines has not yet been made using any of these approaches. This is because these display panels require a refresh cycle with a frame frequency of 60 Hz or higher to prevent flicker. Also, this requirement in turn leads to a single-line scanning time of 10-50 .mu.sec or shorter, which, in such a system, is too short to provide, good contrast.
With a CRT, an image formed on a fluorescent screen persists for a certain time due to the fluorescence. In a TN-type LCD (twisted nematic-type liquid crystal device), an image is formed by utilizing transmittance changes effected by an application of a sufficient driving voltage. In both types of device, it is necessary to use a sufficiently high frame frequency. For both types, the required frame frequency is generally considered to be 30 Hz or higher. The common scanning processes or modes at present include interlaced scanning (skipping one or more lines after each line, e.g., scanning only the odd-numbered or only the even-numbered lines), and non-interlaced scanning process (with no skipping). Other scanning processes include the pairing process and a process comprising simultaneous and parallel scanning of different portions of the picture screen (the latter process is restricted to LCD's). The NTSC standard system calls for interlaced scanning of 2 fields per frame and a frame frequency of 30 Hz, wherein the horizontal scanning time is about 63.5 .mu.sec and about 480 scanning lines are used (in the effective display area). The TN-type LCD has generally used a non-interlaced system including 200-400 scanning lines and a frame frequency of 30 Hz or higher. For use with a CRT, a non-interlaced scanning system with a frame frequency of 40-60 Hz and 200-1000 scanning lines has also been employed.
A typical CRT display or TN-type LCD may comprise 1920 (number of scanning lines).times.2560 (pixels per line)=4,915,200 pixels. In the case of an interlaced system using a frame frequency of 30 Hz, the horizontal scanning time is about 17.5 .mu.sec and the horizontal dot clock frequency is about 147 MHz (without taking into consideration the horizontal flyback time required in a CRT). In the case of a CRT, a horizontal dot clock frequency of 147 MHz leads to a very high beam scanning speed which far exceeds the maximum electron beam modulation frequency of the beam guns used in picture tubes available at present, so that accurate image formation cannot be effected, even by scanning at 17.5 .mu.sec per line. In the case of a TN-type LCD, driving a total of 1920 scanning lines corresponds to a duty factor of 1/1920, which is much lower than the minimum usable duty factor of about 1/400 now possible so that the desired display cannot be achieved. On the other hand, if driving at a practical horizontal scanning speed is used, the frame frequency becomes lower than 30 Hz, and flickering impairs the display quality. For these reasons, enlargement and densification of the picture obtainable with CRT's and TN-type LCD's has been limited because the number of scanning lines cannot be sufficiently increased.
In recent years, Clark and Lagerwall (U.S. Pat. No. 4,367,924) have proposed a ferroelectric liquid crystal (FLC) device having both a high-speed responsive characteristic and a memory characteristic (bistability, the property of assuming either a first optically stable state or a second optically stable state, depending on an applied electric field, and retaining the resultant state in the absence of an electric field). The ferroelectric liquid crystal device shows a chiral smectic C phase (SmC*) or H phase (SmH*) in a specific temperature range, and in this state, shows bistability. The FLC device shows quick response to changes in the applied electric field and is therefore expected to be widely used as a high speed memory-type display device.
A ferroelectric liquid crystal is capable of being used in a large, high-resolution display which surpasses the above-described display apparatus remarkably. In view of its low frame-frequency drive, it is provided with a partial rewriting scanning scheme (in which the only scanning lines rescanned at a given time are those in a region to be overwritten, viz., where previously displayed information is to be replaced with other information) utilizing a memory characteristic in order to provide a man-machine interfacial display apparatus. Such partial rewriting scanning has been disclosed, e.g., in commonly-assigned U.S. Pat. No. 4,655,561 to Kanbe, et al. (the entire disclosure of which is incorporated herein by reference).
A flat panel display comprising a number of pixels equal to 1920 (number of scanning lines).times.2560 (pixels per line) has been achieved using the bistability effect of ferroelectric liquid crystals.
In a line by line scanning scheme for a ferroelectric liquid crystal display (FLCD), the frame refresh frequency decreases as the number of scanning line increases. For example, the frame frequency for an FLCD with a scanning time of 50 .mu.sec/line is: EQU 1920 lines.times.50 .mu.sec/line=96 msec=10 Hz.
On the other hand, it is a very important factor for the operability of computers that the speed for real-time response, the smoothness for the pointing device movement and the keyboard input response time be sufficient for normal use. The pointing devices and character inputs are relatively small in terms of their display area, but require a higher response rate for them each to be properly displayed. For instance, a mouse is written normally at a rate of 60 Hz, and the character input is written at a rate of 30 Hz. Therefore, a frame frequency of 10 Hz is not sufficient for normal operation. The use of the aforementioned "partial rewriting scanning technique" enables the display to rewrite only the portion of the display where new information is to be shown. As a result, the time required for updating the displayed information is reduced. For example, if the mouse font is defined by 32.times.32 bits data, the speed for displaying that data is: EQU 32 lines.times.50 .mu.sec./line=1.6 msec.=625 Hz.
However, actually to use this partial rewriting scanning technique, it is necessary to recognize the partial-rewriting requests and to indicate to the display apparatus the number of lines to be rewritten. Moreover, it involves other procedures relating to the display requests, such as saving background data which are to be overlaid. The processing time for these procedures depends upon the contents of the partial rewriting scanning. It should be noted, however, that the actual frequency for the partial rewriting cannot reach 625 Hz. In general, the processing time for the partial rewriting is proportional to the size of the area to be rewritten, i.e. the number of scanning lines in that area. In the case of a computer terminal display apparatus used as a human interface, there arises the following problem in its functionality.
FIGS. 8A and 8B show an example of a conventional display scheme. In FIG. 8A, three windows 1, 2, 3 are open in the display. In window 1 a first task generates a moving clock, to display the time. In window 2 a second task generates a rotating line (rotating in the direction of the arrow). A text editor is displayed in window 3. Also, a mouse font (large arrow) is moved from one location to another on the display (in the direction of the thin diagonal arrow). FIG. 8B illustrates the time sequence of the generation of the drawing commands (on the left) and the time sequence of the execution of those commands (on the right). As illustrated, the movement of a pointer by the operator moving a mouse is not synchronous with the display. In the top box in each column of FIG. 8B, the mouse is positioned and drawn on the display. Several successive commands (eight, in this example) to draw a line in window 2 are then received, and are executed one after the other. After the eighth line-drawing command is generated, the mouse is moved and a new command to draw it is generated. This command is carried out once the preceding line-drawing commands have been executed, but the execution of each box on the right figure takes the aforementioned processing time, and, as a result, the mouse is not drawn at its new position until quite some time after the operator moved it. The operator, however, is expecting the pointer to move at the same time as he or she moves the mouse, i.e., substantially in real time. This characteristic not only is very disconcerting to the operator, but leads to a tendency for incorrect input by the operator, or at the least, to an appearance in slowness of the system.